Backward post-execution software debugger

ABSTRACT

A method finds an error in a computer program. A plurality of execution breakpoints are set in the computer program. A portion of the execution of the computer program is simulated as recorded in the trace data in the reverse order until one a plurality of conditions is met, wherein one of the plurality of conditions is an attempt to execute a machine instruction associated with one of the plurality of execution breakpoints.

RELATED APPLICATIONS

This Application is a continuation of U.S. patent application Ser. No.10/897,749, filed Jul. 23, 2004, entitled BACKWARD POST-EXECUTIONSOFTWARE DEBUGGER, now U.S. Pat. No. 8,136,096. This Application isrelated to U.S. patent application Ser. No. 10/897,757, filed Jul. 23,2004, entitled FORWARD POST-EXECUTION SOFTWARE DEBUGGER, now U.S. Pat.No. 8,271,955; U.S. patent application Ser. No. 10/897,675, filed Jul.23, 2004, entitled POST-EXECUTION SOFTWARE DEBUGGER WITH ACCESSBREAKPOINTS; U.S. patent application Ser. No. 10/897,750, filed Jul. 23,2004, entitled POST-EXECUTION SOFTWARE DEBUGGER WITH EVENT DISPLAY, nowU.S. Pat. No. 8,132,159; U.S. patent application Ser. No. 10/897,752,filed Jul. 23, 2004, entitled POST-EXECUTION SOFTWARE DEBUGGER WITHPERFORMANCE, now U.S. Pat. No. 7,653,899; U.S. patent application Ser.No. 10/898,457, filed Jul. 23, 2004, entitled POST-EXECUTION SOFTWAREDEBUGGER WITH COVERAGE DISPLAY, now U.S. Pat. No. 8,015,552; and U.S.patent application Ser. No. 10/897,989, filed Jul. 23, 2004, entitledSYSTEM AND METHOD FOR FORWARD AND BACKWARD DEBUGGING USING A TRACE LOG,all of which were filed concurrently by Michael Lindahl, Andre Yew,Morgan Green, Michael Johnson, Craig Franklin, Daniel O'Dowd, and NeilPuthoff. The contents of each of the foregoing applications isincorporated by reference herein in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The disclosure generally relates to computer programs executed oncomputing devices, and more particularly to debuggers for detectingerrors in the computer programs.

2. General Background

Software debuggers have assisted many computer programmers in findingerrors in their computer programs. These errors are generally referredto as “bugs”. The most common bug is a computer program not doing whatit is supposed to do. Finding bugs can often be a cumbersome task for acomputer programmer.

One of the difficulties is that a bug may manifest itself long after thecomputer instruction that caused it was executed. For instance, a firstcomputer instruction may perform a calculation to output a result thatis not used by a second computer instruction until millions ofinstructions after the first instruction was performed.

If the bug is a “reproducible bug”, a computer programmer can usuallyuse a debugger to quickly determine the immediate cause of the bug. Oneof ordinary skill in the art will recognize the term “reproducible bug”to mean a bug that manifests itself in the same way every time a programis executed with the same input.

A debugger can be used to help determine the cause. However, traditionaldebuggers have limited capabilities. Using a traditional debugger oftentakes a programmer days or weeks to find bugs that take a long time toreproduce or that are not reproducible.

SUMMARY OF THE DISCLOSURE

A method and system debug a computer program by using trace data, whichis a recording of the sequence of machine instructions executed by aprogram during a time period along with the addresses and values ofmemory locations accessed and modified by each machine instruction.After the time period, the method and system use the trace data tosimulate the execution of the program during the time period under thecontrol of a debugger. In addition, the method and system use the tracedata to simulate the execution of the program during the time periodbackwards in time under the control of the debugger. In addition, themethod and system use the trace data to display information aboutoperating system events that occurred during the time period. Inaddition, the method and system use the trace data to displayperformance and code coverage data about machine instructions, sourcelines, functions, and tasks during the execution of the program in thetime period.

In one aspect, there is a method of finding an error in a computerprogram. A plurality of execution breakpoints are set in the computerprogram. A portion of the execution of the computer program is simulatedas recorded in the trace data in the reverse order until one a pluralityof conditions is met, wherein one of the plurality of conditions is anattempt to execute a machine instruction associated with one of theplurality of execution breakpoints.

In another aspect, there is a method of finding an error in a computerprogram. A sequence of machine code instructions performed by aprocessor as trace data are recorded. A plurality of breakpoints areset. Each of the plurality of breakpoints is associated with a sourcecode instruction. The plurality of breakpoints are translated into aplurality of translated machine code instructions. A backward executionis simulated of at least a portion of the sequence of machine codeinstructions until an attempt is made to simulate execution of one ofthe translated machine code instructions or the execution of the firstmachine code instruction in the portion of the sequence of machine codeinstructions has been simulated.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, reference will now be made to the accompanyingdrawings.

FIG. 1 illustrates a block diagram of one embodiment in which a computerinteracts with a debugger and a software simulator.

FIG. 2 illustrates an embodiment in which the debugger is stored on thesame computer as the program.

FIG. 3A illustrates an internal schematic diagram of the block diagramillustrated in FIG. 1.

FIG. 3B illustrates a configuration, which maps virtual memory addressesto physical memory addresses.

FIG. 3C illustrates a process for simulating the execution of thecomputer program as recorded within the trace data.

FIG. 4 illustrates an event analysis configuration in which the tracedata that is gathered by the trace module or other recording mechanismcan be used to display information about events.

FIG. 5 illustrates an event debugging system.

FIG. 6A illustrates an event display that can be viewed by a userdebugging the program.

FIG. 6B illustrates how the debugger can analyze multiple tasks.

FIG. 7 displays a process for debugging an event display.

FIG. 8 illustrates a performance display configuration which displaysperformance data on the output device.

FIG. 9 illustrates a performance display that can be displayed on theoutput device.

FIG. 10 illustrates a process for displaying a performance display.

FIG. 11 illustrates a coverage analysis configuration.

FIG. 12 illustrates a coverage display that is displayed on the outputdevice.

FIG. 13 illustrates a process for displaying coverage analysis data.

DETAILED DESCRIPTION

A method and a system are disclosed for improving software debuggingcapabilities. For instance, in one embodiment, a software debugger'scapabilities are enhanced to better handle reproducible bugs thatrequire a long time to reproduce. In another embodiment, a softwaredebugger's capabilities are enhanced to better deal with irreproduciblebugs and bugs that take a long time to reproduce. An irreproducible bugis a bug that produces different behavior each time the program isexecuted. Irreproducible bugs are the most time-consuming andchallenging bugs to find. With the method and system as described below,irreproducible bugs often can be found much more quickly helping toeliminate random bugs, glitches, and crashes that plague many complexprograms.

In yet another embodiment, the method and system help discover bugs thatwill not manifest themselves when the software debugger is connected.For instance, some bugs only manifest themselves when the program runsat full speed. When a debugger is utilized, the program may run at amuch slower speed than normal execution speed. Accordingly, in oneembodiment, the method and system described below provide the ability tomore easily discover this type of bug.

The ability to discover a particular type of bug is not limited to aparticular embodiment. Rather, one embodiment may be helpful in findingone or more of the different types of bugs discussed.

FIG. 1 illustrates a block diagram of one embodiment of a post-executiondebugging environment 100 that allows a computer programmer to debug acomputer program 101. A target computer 102 stores the computer program101 in a memory 111. The target computer 102 also has a processor 110that can read and/or write to the memory 111. In one embodiment, thetarget computer 102 is operably attached to an input device 106. Inanother embodiment, the target computer 102 communicates with the inputdevice 106 through a sensor without being attached to the input device106. In another embodiment, the target computer 102 is operably attachedto an output device 108. In another embodiment, the target computer 102communicates with the output device 108 through a sensor without beingattached to the input device 108. The target computer 102 is essentiallythe computer on which a user would normally run the computer program101.

The post-execution debugging environment 100 also includes a hostcomputer 124 that includes a debugger 112 for debugging the computerprogram 101. The debugger 112 is stored in a memory 120. A softwaresimulator 114 is also stored in the memory 120. The host computer 124also has a processor 118 that reads and/or writes to the memory 120. Inone embodiment, the host computer 124 has an output device 116. Inanother embodiment, the host computer 124 has an input device 122.

In order to perform a particular function, the computer 102 will run theprogram 101 which is a collection of machine instructions and data. Theprogram 101 is stored in a plurality of memory locations in the memory111.

Most programs are written by programmers in a high level programminglanguage. One of ordinary skill in the art will recognize that a highlevel programming language is either a software programming language,such as C, C++, Ada 95, Java, or a hardware description language, whichdescribes the operation of a hardware device, such as Verilog or VHDL.The program as written by the programmer is called source code. Thesource code includes lines of text called source lines.

A compiler is a software program that can be stored on the computer 102that is used to translate source code written in a high levelprogramming language into the machine instructions stored in the memory111 to be executed by the processor 110. The programmer is usually noteven conscious of the existence of these low level machine instructions.The programmer prefers to think of the program in terms of the highlevel source lines in which the software program is written. Thehardware description language can be compiled into either a hardwaredesign or into instructions that can run on a hardware descriptionlanguage simulator. In one embodiment, the compiler is stored on thecomputer 124. In another embodiment, the compiler is stored on a thirdcomputer distinct from the computer 102 and the computer 124.

The processor 110 accesses the memory 111 to execute the machineinstructions of the program 101, which are stored in the memory 111. Inone embodiment, the processor 110 executes each of the machineinstructions one at a time at a rate of many millions of instructionsper second. In one embodiment, the processor 110 has one or moreregisters, which are high speed memory locations physically located inthe processor 110. One of ordinary skill in the art will recognize thatthe processor 110 can access data in a memory location in the memory111, data in a register, or data in both. Further, one of ordinary skillin the art will recognize that the processor 110 can either read datafrom the memory 111 and/or write data to the memory 111.

In one embodiment, the processor 110 and the memory 111 are housedinside of a housing. In another embodiment, the computer 102 is meant tocover any device that has the processor 110 embedded within it. Forinstance, a household appliance that has an embedded processor would bedeemed to have the computer 102. In addition, an automobile, jetairplane, space shuttle, helicopter, train, or any other transportationvehicle with an embedded processor would be deemed to have the computer102. Further, any communications device, such as a cellular telephone,is deemed to be the computer 102. In addition, a personal digitalassistant would be deemed to be the computer 102. One of ordinary skillin the art will recognize that the computer 102 is meant to cover futuredevices that are operably connected to the processor 110. The term“computer” is generally meant to cover any device with the processor110. The memory 111 can be internal, external, or even at a remotelocation. The input device 106 and the output device 108 are notnecessary for the computer 102.

In one embodiment, the processor 110 is a physical chip. In anotherembodiment, the target computer 102 is a software program that emulatesa physical computer. In essence, the processor 110 in this embodiment ispart of the program 101.

The computer 102 receives one or more external inputs from the inputdevice 106. The external inputs can be instructions and/or data to beprocessed by the processor 110. In one embodiment, the input device 106receives one or more external inputs from a human operator. In anotherembodiment, the input device 106 receives one or more external inputsfrom another computer. In yet another embodiment, the input device 106is not needed because a set of pre-inputted data is stored on thecomputer 102 and can be used in an automated fashion, such as accordingto a clock.

The input device 106 can include a variety of hardware configurations.In one embodiment, the input device 106 is a keyboard. In anotherembodiment, the input device 106 is a touch screen. In anotherembodiment, the input device 106 is a computer mouse. In yet anotherembodiment, the input device 106 is a microphone that receives a voicecommand, which is provided to a voice recognition software module. Inone embodiment, the input device 106 is a communications device, such asa modem, which receives data from another computer.

The computer 102 is also able to send data from inside the computer 102to outside the computer 102 through the output device 108. The data thatis outputted by the output device 108 can be a result of the executionof the processor 110. In one embodiment, the output device 108 is adisplay. For example, a screen such as a liquid crystal display (“LCD”),or a plasma display, can be the output device 108. In anotherembodiment, the output device 108 is a printer. In yet anotherembodiment, the output device 108 is an audio speaker, which outputsdata through an audio signal. In another embodiment, the output device108 is a modem that outputs data to another computer through acommunications link.

Any of the different embodiments or variations discussed above withrespect to the computer 102 are also applicable to the computer 124.Further, any of the different embodiments or variations discussed abovewith respect to the input device 106 are also applicable to the inputdevice 122. Finally, any of the different embodiments or variationsdiscussed above with respect to the output device 108 are alsoapplicable to output device 116.

The debugger 112 is a program that helps a programmer find a bug in thetarget program 101. In one embodiment, the debugger 112 allows aprogrammer to interactively find bugs in the program 101 by controllingthe program's 101 operation. In one embodiment, the debugger 112 isstored on the host computer 124 in the memory 120. Usually the debuggingenvironment is displayed on the output device 116. In some instances,the programmer debugs by modifying the state of the program. One ofordinary skill in the art will recognize that the state of a program isa set of values stored in the memory locations of the memory 111 at atime period in the execution of the program. In one embodiment, theprogram 101 that is controlled by the debugger 112 is stored on thecomputer 102. The computer 124 communicates with the computer 102 toallow the debugger 112 to control the program 101. In one embodiment,the host computer 124 is operably attached to the target computer 102.For example, the host computer 124 is operably attached to the targetcomputer 102 through a serial connection, an Ethernet connection, or aUSB connection. In another embodiment, the computer 124 communicateswith the computer 102 through a network connection. In yet anotherembodiment, the computer 124 communicates with the computer 102 througha wireless connection. In one embodiment, the computer 124 communicateswith the computer 102 through a signal, such as an infrared signal.

The debugger 112 has the capability to control, examine, and/or modifythe target program 101, the processor 110, the memory 111, and ingeneral the target computer 102. The debugger 112 controls the targetprogram 101 by providing one or more commands to the target program 101.

In one embodiment, a user enters debugging commands on the host computer124 to debug the program 101 on the target computer 102. The user caninput the commands through the input device 122 as illustrated inFIG. 1. The user can input the commands by inputting characters,clicking icons, clicking buttons, making movements of a computer mouse,providing voice commands, or any other form of input recognized by oneof ordinary skill in the art. The debugger 112 includes controlcommands, display commands, and modification commands for debugging thetarget program 101. One example of the debugger 112 is the MULTIintegrated development environment from Green Hills Software, Inc.However, the debugger 112 is not limited to that particular product.

The control commands of the debugger 112 include a start command tostart execution of the target program 101 and a stop command to stopexecution of the target program 101. The debugger 112 allows the user toplace an execution breakpoint on a machine instruction or a source lineof the target program 101. Whenever the program attempts to execute amachine instruction or a source line that corresponds to an executionbreakpoint, the debugger 112 will provide a stop command to the program.In other words, the debugger 112 stores instructions associated withbreakpoints and checks each instruction prior to execution to see if theinstruction is associated with an execution breakpoint. To get into thevicinity of a bug, a programmer may place breakpoints in the vicinity oflocations where the programmer suspects the bug to be. When the programstops at one of the breakpoints, the programmer can display the state ofthe program 101 to determine if the bug has occurred or is about tooccur. If the cause of the bug is not yet obvious, the programmer caninput a start command to the debugger 112, which then issues a startcommand to the program to continue until the next breakpoint.Alternatively, the programmer can set more breakpoints before continuingexecution.

In another embodiment, the debugger 112 does not need to store theinstructions associated with the breakpoints. After the user sets thebreakpoints, the debugger 112 searches through the compiled machine codeand places a marker at each of the instructions associated withbreakpoints to indicate that the execution should be halted if anattempt is made to execute that particular instruction. A variety ofdifferent configurations can be used for implementing breakpoints.

A computer programmer can often expedite the debugging process byutilizing breakpoints. For instance, breakpoints can be helpful to thecomputer programmer trying to debug a reproducible bug. The computerprogrammer can use a binary searching methodology to determine theoriginal cause of the reproducible bug. The computer programmerinitially determines the immediate cause of the error. Subsequently, thecomputer programmer attempts to surround the immediate cause of theerror with a before-breakpoint and an after-breakpoint. Thebefore-breakpoint is placed at a location at which the computerprogrammer is sure stops the program before the immediate cause of theerror occurs. The after-breakpoint is placed at a location at which theprogrammer is sure stops the program after the immediate cause of theerror occurs. The program is restarted and run to the before-breakpoint.The programmer then sets a new middle-breakpoint about halfway betweenthe before and after-breakpoints. The program is then run to themiddle-breakpoint. If the error has manifested itself, the error isbetween the before-breakpoint and the middle-breakpoint. The process isrepeated with the middle-breakpoint as the after-breakpoint. On theother hand, if the error has not manifested itself at themiddle-breakpoint, the error is between the middle-breakpoint and theafter-breakpoint. The process is then repeated with middle-breakpoint asthe before-breakpoint. In either case, the immediate cause of the bughas now been isolated to an interval that is only about half the lengthof the previous interval. This process may be repeated until theinterval is reduced to just one source line.

Once the source code responsible for the incorrect behavior has beenidentified, the bug may or may not be obvious. Sometimes the identifiedsource code is not at fault. Rather the incorrect behavior may be aresult of incorrect data. The programmer then needs to track down thesource code that last modified the data. The use of read accessbreakpoints, write access breakpoints, and conditional breakpoints canhelp a programmer track down source code that has affected incorrectdata.

A read access breakpoint is a breakpoint that is associated with one ormore memory locations rather than an instruction. The read accessbreakpoint will stop the program 101 whenever the program 101 attemptsto access one of the memory locations associated with the read accessbreakpoint.

A write access breakpoint is a breakpoint that is also associated withone or more memory locations rather than an actual instruction. Thewrite access breakpoint will stop the program whenever the programmerattempts to change one of the memory locations associated with the writeaccess breakpoint. After the programmer determines that a bug is causedby data corruption rather than an incorrect instruction, the programmercan set a write access breakpoint associated with the corrupted memory.The programmer restarts the program with the breakpoint enabled. Eachtime the memory is changed, the program stops and the programmer candetermine if the location at which the program stops is the location inthe source code that corrupted the data.

A programmer often suspects that a bug is in a certain part of thesource code, but the bug only manifests itself after executing machinecode associated with that source code hundreds or thousands of times.Putting a breakpoint in the source code in this instance is impracticalbecause it would require hundreds or thousands of commands to reach thelocation causing the error. A conditional breakpoint helps alleviatethis problem. The conditional breakpoint is a breakpoint that associatesa mathematical formula or a small amount of source code with abreakpoint. When the program reaches the conditional breakpoint, thedebugger evaluates the mathematical formula or the source code to see ifa certain value is produced. If the condition is not true, the debugger112 automatically continues the executing the program without programmerintervention.

The debugger 112 also has display commands to display the state of thetarget program 101. The programmer can provide a display command to thedebugger 112 to view the contents of a particular memory location at aparticular point in the execution of the program. For instance, theprogrammer can view a variable stored in the memory 111 when theexecution of the program 101 is stopped at a breakpoint.

The software simulator 114 is a program that simulates the operation ofthe execution of the program, which is stored on the computer 124 in thememory 120, by the processor 110. In one embodiment, the softwaresimulator 114 simulates the execution of the program at a differentspeed than that which was performed by the processor 110. The softwaresimulator 114 can communicate with the target computer 102 in any of thedifferent embodiments or variations discussed with respect to thecommunication between the debugger 112 and the target computer 102.

FIG. 2 illustrates an embodiment in which the debugger 112 is stored onthe same computer 130 as the program 101. In one embodiment, the memory120 stores the debugger 112, the software simulator 114, and the program101. The computer 130 is essentially both a host computer and a targetcomputer. An input device 122 and/or an output device 116 can beoperably attached to the computer 130 according to the differentembodiments and variations as discussed above with respect to the targetcomputer 102 and the host computer 124. In one embodiment, a copy of theprogram 101 stored on the target computer 102 is made and placed on thehost computer 124.

In one embodiment, the software simulator 114 simulates a theoreticalcomputer system, such as a Java Virtual Machine. In another embodiment,the software simulator 114 is a hardware description language simulator,such as Verilog and VHDL.

FIG. 3A illustrates an internal schematic diagram of the block diagramillustrated in FIG. 1. In one embodiment, the computer 102 has theprocessor 110 interacting with the memory 111 to execute the program101. The processor 110 reads and writes to memory locations in thememory 111. The program 101 includes a plurality of instructions 302.The debugger 112 controls execution of the program 101 to find an errorin the program 101.

In one embodiment, the computer 102 is configured to output a detailedrecord of changes to the processor 110 and the memory 111 as the changesoccur. This detailed record is known as trace data. When the computer102 is halted because of an error, the state of the processor 110 andthe memory 111 can be recorded in the trace data for analysis at a latertime. In one embodiment, the trace data includes an ordered sequence ofmachine instructions performed by the processor 110 during the executionof the program 101. The trace data is helpful to the programmer becauseit is often not obvious from the source code alone the order in whichthe processor 110 actually performed machine instructions translatedfrom the source lines by the compiler or the number of times that anindividual machine instruction was performed. For instance, the sourcecode may have source lines at the very beginning indicating that avariable should be modified if a condition is met. Although theseinstructions appear at the beginning of the source code, theinstructions will not appear at all in the trace data if the conditionis never met.

As another example, the source code may have source lines for printing avalue of a variable within the body of a loop. In this example, thesource lines for printing the value of the variable appear only once inthe source code. On the other hand, the trace data provides a listing ofthe actual number of times that the print execution was executed by theprocessor 110 as a result of the loop.

In another example, the source code has source lines that define afunction at the beginning of the source code. The source code also hassource lines at the end of the source code that make a function call tothe function. Therefore, the source code lists the function well beforethe function call. However, the processor 110 actually executes machineinstructions for the function call before machine instructions for thefunction and executes both sets of machine instructions in fairly closeproximity to one another. The trace data reflects the actual sequence ofmachine instructions that were performed by the processor 110.

The machine instructions are stored in the memory 111. The processor 110then accesses the memory 111 to retrieve the machine instructions andperform a read and/or write instruction to a memory location in thememory 111. In one embodiment, the trace data includes a listing ofmemory addresses. The memory address can either store a machineinstruction or a value or a variable that was accessed or modified bythe machine instruction.

In one embodiment, a trace recordation module 212 records the trace dataoutputted from the processor 110. The trace recordation module 212stores at least a portion of the trace data in a trace data listing 213.The trace data listing 213 lists a plurality of memory instructions 216storing machine instructions that were executed by the processor 110 inthe order in which they were executed. The instruction addresses arememory addresses that store the machine instructions. The machineinstructions may be executed more than once and will therefore appearmore than once in the trace data listing 213. Further, the trace datalisting 213 lists the time that each of the machine instructions in theplurality of memory addresses 216 was executed by the processor 110. Inone embodiment, the trace data listing 213 also lists a plurality ofmemory addresses which each of the machine instructions accesses.Further, there is an indication in the trace data listing 213 that aread took place. For instance, a symbol such as an “R” can be used toindicate a read. In another embodiment, the trace data listing 213 alsolists a plurality of memory addresses which each of the machineinstructions writes to. For instance, a symbol such as a “W” can be usedto indicate a write.

In one embodiment, the trace recordation module 212 provides the tracedata listing 213 to a trace analysis module 214 which composes adatabase 310 from the trace data listing 213. In one embodiment, asingle module can perform the functions of both the trace recordationmodule 212 and the trace analysis module 214. The database 310 storesthe values of the memory locations in the memory 111 that were read orwritten to at each time that the processor 110 executed an instructionin the trace data. When initially composing the database 310, the traceanalysis module 214 leaves the memory locations that were not read orwritten to as unknown values. The trace analysis module 214 can fill inthese unknown values into the database 310 so that the database 310stores the values of each memory location in the memory 111 at a giventime period.

A few different methodologies can be used for filling in the unknownvalues in the database 310. In one embodiment, the trace analysis module214 reviews the database 310 to determine when read instructions wereprocessed. A read instruction at a given time will give the value storedin a memory location of the memory 111 at a given time. The traceanalysis module 214 can then review the memory location at a time periodafter the read in the database 310 to determine if the memory locationhas an unknown value. If the memory location has an unknown value andthere were no writes after the read, the trace analysis module 214stores the value resulting from the read at the memory location with theunknown value. The stored value now likely represents the actual valueof the memory location at the given time because there were not anymodifications to the memory location a time subsequent to the readinstruction.

In yet another embodiment, the trace analysis module 214 searchesthrough the trace data listing 213 to determine if the memory locationthat has an unknown value was written at a time previous to the unknownvalue. If the memory location was written to at an earlier point, theunknown value of the memory location is replaced with the previouslywritten value.

In another embodiment, the trace analysis module 214 stores the valuesof all of the memory locations in the memory 111 at the time that theprocessor 110 halts. By having all of the values of the memory locationsin the memory 111, the trace recordation module 212 can search throughthe database 310 to determine if there are any unknown values. If therewere any unknown values, the trace module would fill in the final statevalue of the memory location as this value likely did not change priorto the final state if there were not any writes to the memory location.In yet another embodiment, the values of the registers located on theprocessor 110 can also be composed for each given time in the database310 by analyzing the trace data listing 213.

In one embodiment, the database 310 is composed from at least a portionof the trace data listing 213. The trace data listing 213 couldpotentially have billions of instructions. Building the database 310 onsuch a large amount of data could potentially involve a very largeamount of memory and a great deal of time. If a smaller portion of thetrace data 213 is used within the vicinity of the location at which theprogrammer surmises to be the immediate cause of the error, theprogrammer can compose the database 310 much more quickly with fewerresources. For example, the programmer can select twenty-five thousandinstructions within one billion instructions in the trace data listing213 as opposed to one billion instructions. If the twenty-five thousandinstructions prove to be insufficient for finding the error, theprogrammer can set the portion of the trace data listing 213 to belarger, such as one hundred thousand instructions. In one embodiment,the database 310 is stored in the memory 120 in the host computer 124.In yet another embodiment, the database 310 is stored in the memory 111in the target computer 102. In another embodiment, the database 310 isstored on a computer readable medium that stores data, such as adiskette, computer memory, hard drive, USB jump drive, or another datastorage device. The computer readable medium can be external to the hostcomputer 124 or the target computer 102.

In one embodiment, the database 310 is not needed. The trace datalisting 213 is simply annotated with the memory values from previous orsubsequent instructions. In addition, a simulation instruction can beannotated to each instruction in the trace data listing 213 to indicatehow to move backwards from one instruction to a previous instruction.

In one embodiment, the trace recordation module 212 is a hardware deviceoperably connected to the processor 110. In another embodiment, thetrace module is part of the processor 110. In yet another embodiment,the trace module is a software program that the processor 110 executesto record the trace data.

In one embodiment, the trace recordation module 212 is an externalhardware device that is outside of the computer 102 such as a logicanalyzer, in-circuit emulator, or a trace probe. The trace recordationmodule 212 monitors the execution of the processor 110 without the needto intervene in the operation of the processor 110.

In yet another embodiment, there is no need for the trace recordationmodule 212. The processor 110 outputs the trace data. In one embodiment,the processor 110 outputs the trace data directly to the softwaresimulator 114. In another embodiment, the processor 110 outputs thetrace data directly to the debugger 112. The debugger 112 can thenoutput the trace data to the software simulator 114 if a simulation ofinstructions in the trace data is to be performed. In yet anotherembodiment, the processor 110 outputs the trace data to the outputdevice 108, which can then output the trace data to the debugger 112,the software simulator 114, or another computer.

In one embodiment, there is no need for the trace module 112 because thesoftware simulator 114 records trace data during the simulation of anexecution of a program. The trace data recorded from the simulation canthen be analyzed.

In one embodiment, the processor 110 transmits the plurality of memoryaddresses 216 storing the machine instructions in the order of executionto the trace recordation module 212. In one embodiment, the plurality ofmemory addresses instructions 216 stored in the trace recordation module212 is a set of memory addresses for machine instructions in the orderactually performed by the processor 110 during the execution of theprogram. Each of the machine instructions in the plurality of memoryaddresses 216 is translated from a source code instruction in theprogram 101. The translation from source code to machine code can beaccomplished through a compiler. In one embodiment, the compiler outputsa file with the translation results. The debugger 112 can access thisfile to determine what source code instruction is associated with eachmachine code instruction. In one embodiment, the debugger 112 includesthe compiler.

The software simulator 114 essentially recreates the state of theprocessor 110 and the memory 111 for a set of different time periods asrecorded by the trace data. The software simulator 114 can use the tracedata to simulate the execution of the next instruction of the program byapplying the changes that the actual execution of the next machineinstruction recorded in the trace data and the software simulator's 114knowledge of the effects of the instruction. In one embodiment, thesimulator 114 accesses the database 310 to recreate a state for a giventime period. For instance, if the simulator 114 is attempting to executea simulation from the beginning of the trace data listing 213, thesimulator 114 would access the database 310 to determine the values ofthe memory locations in the memory 111 at time t₁ as illustrated in thedatabase 310. The simulator 114 would then set corresponding simulatedmemory locations to these values.

In one embodiment, the debugger 112 controls the simulator 114. Thesimulator 114 represents the approximate state of the program 101 atsome time t₁ that is represented in the trace data listing 213. If thedebugger 112 provides a run command to execute the machine instructionat time t₁, the simulator 114 sets the corresponding simulated memorylocation to the values stored in the database 310 at time t₁. When thedebugger 112 provides a run command to the simulator 114, the simulator114 will begin simulating execution of the machine instructions from thestate at that point. Therefore, the simulator 114 initially representsthe state of the program 101 at some time and simulates the execution ofthe next instruction by representing the state of the program at thetime of the next instruction.

For example, the simulator 114 can represent the state of the program101 at the time t₁ and can simulate the execution of the nextinstruction at time t₂ by representing the state of the program 101 att₂. Accordingly, the simulator 114 sets the corresponding simulatedmemory locations to the values stored in the database 310 at time t₂. Ineffect, the simulator 114 is simulating the change to the state of thesystem that the machine instruction at time t₂ caused. In oneembodiment, the simulator 114 modifies the simulated memory locations attime t₁ so that the simulated memory locations at time t₂ reflect thememory values stored at time t₂ in the database 310. The simulator 114can represent the state at each subsequent machine instruction and stopwhen a breakpoint or the end of the trace data listing 213 is reached.

If the debugger 112 provides a run backwards command to the simulator114, the simulator 114 represents the state of the program 101 at aprevious time. For instance, if the simulator 114 is currentlyrepresenting the state of the program 101 at time t₁₀₀ and the debugger112 provides a run backwards command, the simulator 114 will representthe state of the program 101 at the time for time t₉₉ for the previousinstruction. Accordingly, the simulator 114 sets the correspondingsimulated memory locations to the values stored in the database 310 attime t₉₉. In essence, the software simulator 114 can undo the effects ofthe execution of the last instruction of the program 101 executed byundoing the changes that the actual execution of that machineinstruction recorded in the trace data.

In one embodiment, the debugger 112 will display data in response todisplay commands when the simulation is halted. The debugger 112 willdisplay the data in response to display commands after a forwardsimulation or after a backwards simulation.

In one embodiment, both the software program 101 and the softwaresimulator 114 are controlled by the debugger 112. The debugger 112provides a familiar interface to analyze the plurality of instructions208 of source code in the program 101. The programmer can place aplurality of breakpoints in the source code of the program 101 just asif the programmer was using breakpoints in a traditional debugger. Oncethe programmer selects an instruction for a breakpoint, the debugger 112can determine what machine instruction correlating to the source codeinstruction would have been executed by the processor 110 at thebreakpoint. The debugger 112 makes this determination by reviewing atranslation table to see how the compiler translated the source codeinstruction into a machine instruction. In one embodiment, thetranslation table is produced at the time that the compiler compiled thesource code into machine instructions. The debugger 112 can then accessthe translation table in a file.

The debugger 112 then provides the memory addresses of the machineinstructions at which breakpoints are to be placed to the softwaresimulator 114. In one embodiment, the simulator 114 searches through theplurality of memory addresses 216 storing machine instructions in thetrace data listing 213 to find the selected memory addresses. In oneembodiment, the debugger 112 searches through the memory addressesreceived in the plurality of memory addresses 216 storing machineinstructions. Once the selected memory address is found, the debugger112 essentially places a breakpoint at that particular memory address inthe plurality of memory addresses 216.

FIG. 3B illustrates a configuration, which maps virtual memory addressesto physical memory addresses. This configuration is helpful in somesystems for halting simulation of execution at an access breakpoint.

In one embodiment, a read access breakpoint can be provided to thesoftware simulator 114. Some systems only have a listing of physicalmemory addresses that are read from and/or written to. Therefore, a usercan select a memory address for a read access breakpoint which thedebugger 112 can provide to the simulator 114. In one embodiment, thesimulator 114 stores the memory address in a list with otherbreakpoints. Accordingly, the simulator 114 can check each instructionthat the simulator 114 simulates prior to simulation to determine if thememory address associated with the read access breakpoint appears as aread in the trace data listing 213 for the given instruction. Forexample, the trace data listing 213 can have an “R”. If the memoryaddress appears in the trace data listing 213, the simulator 114 haltsexecution prior to the simulation of the execution of the giveninstruction.

In one embodiment, a write access breakpoint can be provided to thesoftware simulator 114. Some systems only have a listing of physicalmemory addresses that are read from and/or written to. Therefore, a usercan select a memory address for a write access breakpoint which thedebugger 112 can provide to the simulator 114. In one embodiment, thesimulator 114 stores the memory address in a list with otherbreakpoints. Accordingly, the simulator 114 can check each instructionthat the simulator 114 simulates prior to simulation to determine if thememory address associated with the write access breakpoint appears as awrite in the trace data listing 213 for the given instruction. Forexample, the trace data listing 213 can have a “W”. If the memoryaddress appears in the trace data listing 213, the simulator 114 haltsexecution prior to the simulation of the execution of the giveninstruction.

However, some computer systems use both physical memory addresses andvirtual memory addresses. Although the trace data normally only listsphysical memory addresses, the debugger 112 that works in conjunctionwith these computer systems will likely provide both physical memoryaddresses and virtual memory addresses to the user. By the term virtualmemory address, one of ordinary skill in the art will recognize that theoperating system pretends either that the memory 111 organizes data in acertain manner or has an additional set of memory locations. Thesevirtual memory addresses are associated with physical memory addresses.In one embodiment, these virtual memory addresses provide information onwhere to find the stored data somewhere else in the memory 111. Inanother embodiment, these virtual memory addresses provide informationon where to find the stored data on a disk. When the processor 110 hasto read or write to a virtual memory address, the processor 110 accessesthe physical memory address that the operating system 320 has associatedwith virtual memory address.

Once the programmer selects a memory location for an access breakpoint,the debugger 112 performs a mapping from a virtual memory address thatcorresponds with a physical memory address. When a programmer providesan access breakpoint to the debugger 112, the programmer is providing avirtual memory address. However, the trace data listing 213 only liststhe physical addresses to which the processor 110 reads or writes.Therefore, the debugger 112 performs a mapping.

In one embodiment, the debugger 112 maps the virtual memory address tothe corresponding physical memory address by looking to the database 310and reviewing the memory addresses for which the operating system 320translation tables are stored. The operating system provides a mappingtable that records what physical memory address the operating systemassigned to a particular virtual memory address. The debugger 112reviews the mapping table in the operating system to translate thevirtual memory address associated with the access breakpoint into aphysical memory address. In one embodiment, the debugger 112 thenprovides the physical memory address to the software simulator 114 whichsets an access breakpoint so that the simulator 114 halts execution eachtime the memory location of interest is accessed.

In one embodiment, the mapping table is stored on an external computerreadable medium. In another embodiment, the mapping table is stored in afile. If the mapping values have been permanently assigned, a file canbe accessed to view the mapping table.

In one embodiment, the access breakpoint is a read access breakpointthat halts execution of the simulation each time the memory address isread from. In another embodiment, the access breakpoint is a writeaccess breakpoint that halts execution of the simulation each time thememory address is written to.

In one embodiment, the trace recordation module 212 inserts thebreakpoints in the database 310 when composing the database 310. Thetrace recordation module 212 receives the physical memory address fromthe debugger 112. For each access of the memory address in the tracedata listing 213, the trace recordation module 212 inserts a marker intothe database 310 to halt simulation. The software simulator 114 thenhalts simulation if there is an attempt by the simulator 114 to set thesimulated memory values to those at the point in the database 310.

In another embodiment, the trace data listing 213 is annotated so thatthe virtual memory address corresponding to the physical memory addressis also listed. The trace recordation module 212 provides the trace datalisting 213 to the debugger 112, which then searches through the tracedata listing 213 to determine what physical memory addresses are presentin the trace data listing 213. The debugger 112 performs a translationfor each of the physical memory addresses into a virtual memory address.In one embodiment, the debugger 112 annotates the trace data listing 213and provides the annotated trace data listing 213 to the tracerecordation module 212. In another embodiment, the debugger 112 providesa list of the translations to be performed and the trace recordationmodule 212 actually performs the annotation of the trace data listing213.

In one embodiment, the debugger 112 provides a run command to thesoftware simulator 114 which simulates the execution of instructionsfrom the current simulated time until the software simulator attempts toexecute a machine instruction in the trace data that has a breakpoint ata memory address which is modified or accessed or until the end of tracedata is reached. In another embodiment, the debugger 112 provides a runbackward command to the software simulator 114 until the softwaresimulator 114 attempts to execute the machine instruction in the tracedata that has a breakpoint at the associated source code instruction oruntil the beginning of the trace data is reached. In another embodiment,the debugger 112 receives a plurality of breakpoints. In one embodiment,the breakpoints in the plurality of breakpoints are different. Forinstance, two different execution breakpoints can be set. Further, anexecution breakpoint and a write access breakpoint can be set. One ofordinary skill in the art will recognize that many different types ofbreakpoints can be used as the plurality of breakpoints. The simulator114 attempts to simulate the execution of the plurality of machineinstructions stored in the plurality of memory addresses 216 until oneof the plurality of breakpoints is reached.

In essence, the debugger 112 provides the user with an interface to arecording of the execution of the processor 110 in a given time period.The user is given the appearance of using a traditional debugger, withwhich he or she is familiar. While the user provides the same commandsto the debugger 112 as the user would provide to a traditional debugger,the debugger 112 is actually navigating through a recordation of theprogram to be debugged rather than controlling an execution of theprogram to be debugged. Accordingly, the actual implementation remainstransparent to the user.

The debugger 112 has more robust capabilities than a traditionaldebugger. Much like a VCR, the debugger 112 allows the user essentiallyto rewind and fast-forward through the recordation of the trace data. Inaddition, the debugger 112 greatly enables a programmer's ability tofind the more difficult types of bugs.

Accordingly, the debugger 112 helps find irreproducible bugs byessentially making the irreproducible bugs reproducible. Much of thedifficulty in finding an irreproducible bug is that the irreproduciblebug rarely manifests itself after the program crashes. At the time ofthe crash, the programmer can either debug the program at its presentstate or restart the program and attempt to use tools such asbreakpoints to determine the bug. The difficulty with debugging in thepresent state when the crash occurs is that there is not muchflexibility for the programmer to analyze previous instructions andprevious memory values prior to the crash. The problem with restartingthe program is that the bug is irreproducible and will likely notmanifest itself again for a very long period of time. The manifestationof the irreproducible bug is essentially lost when the program isrestarted.

The trace module's 212 recordation of the execution of the processor 110effectively prevents the manifestation of the irreproducible bug frombeing lost. The state of the program at the time of the crash and attimes previous to the crash is reflected in the trace data. There is astrong likelihood that the irreproducible bug is somewhere in the tracedata. The programmer can perform forward and backward simulations untilthe bug is reached.

In one embodiment, the debugger 112 provides the programmer with theability to perform an enhanced binary search, which traditionaldebuggers are not capable of performing. A traditional debugger requiresthe programmer to set breakpoints in the source code and then restartthe program from the very beginning. The program then executes until oneor more breakpoints is reached and then presumably crashes or producesan undesirable result somewhere in between the breakpoints. Theprogrammer then resets some breakpoints to cover this narrower range andmust restart the program again and execute the program from the verybeginning to the new set of breakpoints.

The enhanced binary searching methodology does not require theprogrammer to restart execution of the program from the very beginningBy navigating through a recordation of an execution as opposed toactually executing, the debugger 112 does not need to restart theprogram when a crash or undesirable result occurs.

In one embodiment, the programmer sets a before-breakpoint at a pointthat the programmer is sure is before the cause of the bug. Theprogrammer then runs the program backward to the before-breakpoint. Theprogrammer than sets an after-breakpoint that is after the manifestationof the bug. The programmer then sets a middle-breakpoint about halfwayin between the before and after-breakpoints. The execution of theinstructions between the before-breakpoint and the middle-breakpoint issimulated. If the error manifests itself between the before-breakpointand the middle-breakpoint, the execution is simulated backwards to thebefore-breakpoint. The middle-breakpoint becomes the after-breakpointand a new middle-breakpoint is set. The execution is then simulatedforward from the before-breakpoint without having to restart theprogram. If the error manifests itself between the middle-breakpoint andthe after-breakpoint, the execution is simulated backwards to themiddle-breakpoint. Accordingly, the middle-breakpoint is then set as thebefore-breakpoint. A new middle-breakpoint is set about halfway inbetween the new before-breakpoint and the after-breakpoint. Accordingly,the execution can be simulated without having to restart the program.Further, the binary search reduces the amount of code to debug by half.

Another type of bug that the debugger 112 helps reproduce is thereproducible bug that takes a long time to reproduce. In order toreproduce a reproducible bug, the same conditions that occurred when thebug manifested itself may be needed. For instance, the manifestation ofthe bug may depend on a particular time of the day. Therefore, aprogrammer using a traditional debugger may take a long time toreproduce a manifestation of the bug. The debugger 112 is able tonavigate through a recording that has the conditions in which the bugmanifested itself. When the bug manifests itself, either the programmerchooses to stop the program or the program stops as a result of the bug.In one embodiment, the trace data is recorded up until the point thatthe bug manifested itself. In one embodiment, the trace data is sent tothe host computer 124 to be processed by the trace recordation module212. The debugger 112 can then be used to perform the enhanced binarysearch to efficiently find the reproducible bug that would otherwisetake a long time to reproduce.

Another type of bug that the debugger 112 can be used to find is the bugthat only manifests itself when the program is run at full speed.Traditional debugging methods simply do not work because as soon as atraditional breakpoint is reached, the program is not executing at fullspeed and the bug may not manifest itself. With the trace recordationmodule 212, a recording of the execution of the processor 110 at fullspeed is taken. The debugger 112 can then navigate through the recordingtaken by the trace recordation module 212 because the bug has beencaptured in the recording. Unlike traditional debuggers, the debugger112 can use breakpoints to find the type of bug because the speed of thesimulation of the execution of the trace data does not affect whetherthe bug appears in the trace data. The bug has been recorded in thetrace data. Accordingly, the programmer can use the debugger 112 to moveforward and backward through the recording, even at slow speeds, to findthe bug. Even though the simulated debugging process simulates runningand stopping the program, the bug manifests itself each time just as itdid when the program was actually run. Therefore, the debugger 112allows bugs that only manifest themselves when run at full speed to bedebugged just as easily as an ordinary bug.

The debugger 112 is also helpful in discerning corrupted data. Sometimesthe instruction at which a bug manifests itself is not the cause of thebug but rather the data that the processor used to perform theinstruction. The data had been corrupted at some earlier source line.This bug may or may not be reproducible. When a programmer encountersthis bug, the programmer can use a traditional debugger to set an accessbreakpoint and restart the program to do a forward execution until theaccess breakpoint is reached. However, the data may not be corruptedduring the programmer's subsequent execution because the bug may be anirreproducible bug or a reproducible bug that takes a long time toreproduce.

Whether or not the bug is an irreproducible bug or a reproducible bugthat takes a long time to reproduce, the debugger 112 allows theprogrammer to efficiently find the cause of the corrupted data. In oneembodiment, the programmer uses the debugger 112 to set a write accessbreakpoint on the corrupted memory location. The debugger 112 thenallows the programmer to simulate an execution backward from themanifestation of the bug until the write access breakpoint is reachedwithout having to restart the program. The simulation can be movedbackward to each instruction at which the write access breakpoint haltsthe simulation of the execution until the bug is found.

The debugger 112 is also helpful in simplifying a programmer's strategyfor where to set breakpoints and how many breakpoints to set. With atraditional debugger, the programmer does not want to set a breakpointtoo far along in the program because then the bug may be passed when theprogram executes. If the bug is passed, the programmer has wasted timeand must restart the program from the beginning. Similarly, if theprogrammer places too many breakpoints to make sure that the bugmanifests itself before one breakpoint, debugging becomes a very slowprocess. The debugger 112 allows the programmer to freely setbreakpoints without being concerned about having to restart the programif the breakpoint is placed past the manifestation of the bug. If thebreakpoint is placed too far into the source code, the programmer canuse the debugger 112 to simulate backward to the last breakpoint, setone or more breakpoints that are not so far along in the source code,and simulate forward to the new breakpoints without having to restartthe program.

The debugger 112 is also helpful for determining a path that the sourcecode took. For instance, a programmer will sometimes place a breakpointinto the code of a function to see if a function call was made to thefunction. However, the source code may have multiple calls to the samefunction. Finding which call involved the function at the breakpoint isnot easy with a traditional debugger. The programmer can analyze thestate of the program or can even set breakpoints at each of the functioncalls and restart the program. The debugger 112 simplifies the processof determining the path selected by the source code. For instance, whena breakpoint is set in a function, the programmer can use the debugger112 to simulate backward to the call that invoked the function.Therefore, the debugger 112 provides a fast and simple way to find thepath selected by the source code.

Although the trace data generally provides an ordered listing of themachine instructions performed by the processor 110, the trace datausually has some imperfect portions. When the amount of trace data thatis being recorded becomes large, overflow may occur. There may be gapsin portions of the trace data. Further, some machine instructions may belisted out of order. Software tools exist to help correct these errors.Even with the errors, the majority of the trace data is reliable.

In one embodiment, the gaps in the trace data can be filled in through acorrection methodology. First, the trace data is analyzed to determineif there are any gaps. When the trace recordation module 212 isrecording the execution of the processor 110, the trace recordationmodule 212 stores the machine instructions and the memory values in atemporary buffer prior to composing the trace data in the trace datalisting 213. In some instances, the temporary buffer becomes full, andthe trace recordation module 212 is unable to record one or more machineinstructions and/or memory values. Although the trace recordation module212 will be unable to record the trace data during this overflow, thetrace recordation module 212 is able to record that an overflow problemoccurred for a given instruction. Therefore, the trace recordationmodule 212 knows what instruction is supposed to be performed, but doesnot have a recording of the execution of the instruction. In oneembodiment, the debugger 112 accesses the database 310 to determine thestate of the program at a time prior to the overflow. The softwaresimulator 114 then determines how the execution of the instruction notperformed as a result of the overflow would affect the memory 111 andregisters of the processor 110. The trace recordation module 212 thenrecords the values in the database 310 for the time period in which theoverflow occurred.

FIG. 3C illustrates a process 350 for simulating the execution of thecomputer program 101 as recorded within the trace data. At a processblock 352, the process records trace data from an execution of thecomputer program 101 for a time period. Accordingly, at a next processblock 354, the process 350 sets a plurality of execution breakpoints ina computer program 101. Finally, at a process block 356, the processsimulates a portion of the execution of the computer program 101, asrecorded in the trace data until one of a plurality of conditions ismet. In one embodiment, one of the plurality of conditions is an attemptto execute a machine instruction associated with one of the plurality ofexecution breakpoints.

FIG. 4 illustrates an event analysis configuration 400 in which thetrace data that is gathered by the trace recordation module 212 or otherrecording mechanism is used to display information about operatingsystem events. An operating system event is an event that requires theoperating system 320 to respond by executing machine instructions toprocess the event.

The memory 111 includes a storage portion 402 that has a series ofaddresses 406 with associated data. A part of the storage portion 402 isdedicated to storing the machine instructions and data of an operatingsystem 320. The operating system 320 is a software program stored in thememory 111 that the processor 110 executes to provide services to theprogram 101. For instance, when the program 101 needs to interact withhardware, such as a disk for input/output (“I/O”), the program 101 caninvoke operating system machine instructions to read or write to disk.The operating system machine instructions are stored in various memorylocations that can be accessed by the associated memory address.

An interrupt is a hardware initiated function call to an operatingsystem function caused by an external event. For instance, when the userenters input through the input device 106, such as a keyboard, the inputdevice 106 will invoke a function call to the operating system toprocess the data that is inputted.

The program 101 may consist of one or more tasks. A task is created andmanaged by the operating system 320. The processor 110 executes a task'smachine instructions one at a time. There may be many tasks andinterrupts but the processor 110 can only execute one task or interruptat a time. All of the tasks and interrupts compete for access to theprocessor 110 to execute their respective machine instructions. For atask to continue its execution all other tasks and interrupts must stopexecuting.

Traditional systems require that additional machine instructions beadded to the operating system in order for information about events tobe gathered. These additional machine instructions invariably affect theperformance and the behavior of the computer system, often to asignificant degree. For instance, the computer system is usually sloweddown, sometimes considerably. Further, the computer system often behavesdifferently than if the event data was not being gathered. The effect ofthis change in behavior can sometimes be large enough to mislead aprogrammer into making incorrect decisions about how to debug theprogram 101.

The event analysis configuration 400 allows event data to be gatheredwithout degrading the performance of the computer 102. The eventanalysis configuration 400 accomplishes this by analyzing trace datainstead of placing additional machine instructions in the operatingsystem. The event analysis configuration 400 includes an event analysismodule 408 that analyzes trace data and gathers event data to bedisplayed. In one embodiment, the event analysis program 400 is thetrace recordation module 212. In another embodiment, the event analysisprogram 408 works in conjunction with the trace recordation module 212.

In one embodiment, the trace data recorded by the trace recordationmodule 212 includes a record of the execution of machine instructionsfrom the operating system as well as the program 101. If the program 101makes an operating system service call, the trace data listing 213 willhave a sequence of machine instructions for the program 101 followed bya sequence of machine instructions associated with operating systemfollowed by more machine instruction from the program 101. In addition,the trace data listing 213 includes the addresses and values of thememory 111 that the operating system accesses and modifies.

In one embodiment, the event analysis program 408 maintains or hasaccess to a selected event list 412. The selected event list 412includes events for which event data is to be displayed. Many eventstake place within a computer system, and the display of event data forevery event might make debugging a confusing process. The user mayselect some of the events that are likely to be of interest fordebugging purposes. The event analysis program 408 places these eventson the selected event list 412.

The event analysis module 408 obtains from the operating system 320debugging information such as the addresses of some or all of themachine instructions in the operating system that service each of theevents listed on the selected event list 412. The event analysis program408 stores the operating system machine instruction addressescorresponding to each event in the selected event list 412. In oneembodiment, a separate table from the selected event list 412 can becreated to reflect the operating system machine instruction that isassociated with each of the selected events.

FIG. 5 illustrates an event debugging system 500. In one embodiment, theevent analysis program 408 accesses the trace recordation module 212 tosearch the trace data listing 213 for operating system machineinstruction addresses that correspond to events listed in the selectedevent list 412. If machine instruction addresses corresponding to anevent in the selected event list 412 are found in the trace data listing213, the processor 110 executed operating system machine instructionsfor a selected event during the time that the trace data records forthose machine instructions.

The event analysis program 408 is programmed with the knowledge of wherein the memory 111 or registers the arguments and other descriptiveinformation about the event is stored at the time of the event. Theevent analysis program 408 then obtains the values of the arguments andother descriptive information about the event by requesting from thedatabase 310 the values of the associated memory and register values atthe time the operating system processed the event.

In one embodiment, a dedicated memory address or register in theoperating system 320 stores an identification of the task that iscurrently running. The event analysis program 408 searches the tracedata listing 213 for writes to this address. Each write to this addressrepresents a change in the task that is currently executing. Using thisinformation, in one embodiment, the event debugging system 500 recordsin the trace data listing 213 which task is executing for each machineinstruction in the trace data. In another embodiment, the eventdebugging system 500 records the task in a separate list. In yet anotherembodiment, the event debugging system 500 records the task in thedatabase 310. In another embodiment, the current task can be determinedby searching the trace data listing 213 for machine instructions fromthe operating system context switch function that is responsible forswitching the task that is executing. The event analysis program 408obtains the identification of the task which begins running byrequesting from the database 310 the value of the registers or memorylocations in which the identification of the task is passed to theoperating system context switch function when the operating systemcontext switch machine instructions are executed.

The event analysis module 408 takes the data that the event analysismodule 408 has extracted and organized into event data and provides theevent data to the output device 108. For instance, the event data can bethe name of the event, the name of the task that executed the event, thetime that the event was executed, and the arguments that were passed tothe operating system code to execute the event. The output device 108can be a graphical user interface that provides a display for the eventdata that allows the user to view and navigate through the event data.In one embodiment, the event analysis program 408 provides the eventdata to the debugger 112 which then provides the event data to theoutput device 108. In another embodiment, the event analysis module 408is a part of the debugger 112. In yet another embodiment, there is noneed for the event analysis module 408 because the debugger 112 performsthe functions of the event analysis module 408.

FIG. 6A illustrates an event display 600 that can be viewed by a userdebugging the program 101. Once the event data is displayed on theoutput device 108, the user can interact with the event data to debugthe program 101. The event display has a horizontal time line 604 thatindicates the time running from left to right across the display. Usingthe time line 604, a viewer can determine the time that an eventoccurred. The event display 600 displays a horizontal line for some orall of the tasks or interrupts that the event analysis program 408determined to be executing in the trace data. Each horizontal linerepresents the execution of the corresponding task at the time indicatedby the time line 604. In one embodiment the line representing the taskis green when the task is executing and black when it is not executing.Various symbols can be used to indicate event data. For instance, ablock 606 can be used to indicate that an event occurred at a particulartime. If an event is associated with one task, such as a task requestfor an operating system service, the event indication appears on or nearthe line associated with the task at the point corresponding to the timeof the event. Task one began executing but stopped temporarily becausean Interrupt began executing. When the Interrupt stopped executing, taskone continued executing. When task one was finished executing, task twobegan executing. In one embodiment, each event has a particular symbolthat indicates the type of event that is being performed. For instance,a triangle may be placed next to task one to indicate that task one isindicating an event to perform I/O. In another embodiment, events can becolor coded to indicate a particular type of event. For instance, taskone can be color coded green to indicated that task one is performing anI/O.

In one embodiment, the user can select one of the events to debug byselecting the event on the output device 108. For instance, the user canuse a mouse pointer to click on the event to select it. This selects thetime of the event and the task associated with the event. In anotherembodiment, the user can select a time and a task by clicking on a taskline at a point that corresponds to a time.

Once the task and time are selected to debug, the debugger 112 initiatesa debugging session for the selected task at the selected time. Thedebugger 112 displays a window with the source code that was executingat the time in the selected task. The debugger 112 allows the user toexamine the value of variables, registers, and memory locations as theywere at that time. The debugger 112 also allows the user to placebreakpoints anywhere in the source code and run the execution of thetask forwards or backwards from that point and utilize the debugger'sother capabilities normally available when debugging a live task.

The event display provides the programmer with the ability to obtain ahigh level view of what went wrong with the execution of the program 101prior to attempting to debug the problem. The event display assists theprogrammer in determining where to begin debugging or placingbreakpoints. The event display provides the programmer with an idea ofwhere the cause of the bug may be. For instance, if the programmerdetermines that a particular task has stopped running for a long periodof time, the programmer can then debug the task shortly before itstopped running to determine the cause. In addition, the programmer cansimulate the execution of the task both forwards and backwards in orderto narrow down the cause of the bug. In one embodiment, the simulator114 accesses the database 310. In another embodiment, the simulator 114accesses the trace recordation module 212, which then accesses anannotated trace data listing 213 that facilitates the simulator 114simulating execution forward and backward. Further, the programmer canuse the modified binary searching method on the execution of the task.

FIG. 6B illustrates how the debugger 112 can analyze multiple tasks.Multiple windows can be displayed, each for debugging a particular task.Each window will only show the execution of the source code for theparticular task associated with the window. The trace data provides thetask context switches and therefore allows each instruction and eachvalue in the trace data to be identified with the task that executed theinstruction. Using the knowledge that the debugger 112 has of which taskexecuted an instruction, the debugger 112 updates the correct windowassociated with the simulation of the execution of the program 101 bythe simulator 111.

For instance, the debugger 112 can receive simulation data from thesimulator 111 to be displayed on the output device 108. For instance,the debugger 112 can then update a task one display with the simulationof a first sequence of instructions, a task two display with thesimulation of a second sequence of instructions, and a task threedisplay with the simulation of a third sequence of instructions.

FIG. 7 displays a process 700 for implementing an event display. At aprocess block 702, the operating system 320 object code is accessed inthe memory 111. Operating system machine instruction addresses forselected events are determined at a process block 704. Further, at aprocess block 706, the trace data listing 213 is searched through forexecution of machine instructions associated with events. The executionof the machine instructions can be found by looking for the memoryaddresses that store the machine instructions in the trace data listing213. At a process block 708, the times at which the events were executedare determined. In addition, at a process block 710 the database 310 issearched through to find task related data at the specified times. At aprocess block 712, the event display is computed. Further, at a processblock 714, user selection of the event to debug is received.

FIG. 8 illustrates a performance display configuration 800, whichdisplays performance data to the output device 108. Performance dataincludes data about the amount of time that the processor 110 executed afunction, task, source line, or instruction during the time representedby the trace data listing 213. The performance data can be aquantitative figure or can be a fraction or percentage of theprocessor's total execution time.

The performance analysis module 806 uses the trace data listing 213 tocreate a table which records for each machine instruction address in theprogram 106 the total amount of time that the processor 110 spentexecuting the machine instruction at that address during the time periodrepresented by the trace data listing 213. Each instruction in theprogram may be executed many times. In one embodiment, for each timethat the instruction was executed, there is an entry in the trace datalisting 213 that records the time that the instruction was executed. Inanother embodiment, there is an entry in another table. The performanceanalysis module 806 processes each machine instruction executionrepresented in the trace data listing 213 by subtracting the executiontime of the next instruction from the execution time of thisinstruction. This represents the amount of time that the processor 110spent executing this instruction. The time spent executing thisinstruction is added to the element of the table that represents themachine instruction address for this instruction. When all of the tracedata listing 213 has been processed the table records for each machineinstruction address how much total time the processor 110 spentexecuting the machine instruction at that address during the executionof the program recorded in the trace data listing 213.

In one embodiment, the performance analysis module 806 accesses amapping file 804 that specifies which machine instruction addressescorrespond to each source code line in the program. The performanceanalysis module 806 creates a table which records for each source linein the program the amount of time that the processor 110 spent executingthat source line during the execution of the program recorded in thetrace data listing 213. The performance analysis module 806 fills ineach source line entry in the table by summing up the time spentexecuting each machine instruction that is part of the source line.

In one embodiment, the performance analysis module 806 accesses amapping file 804 that specifies which machine instruction addressescorrespond to each function in the program. The performance analysismodule 806 creates a table which records for each function in theprogram the amount of time that the processor 110 spent executing thatfunction during the execution of the program recorded in the trace datalisting 213. The performance analysis module 806 fills in the entry foreach function in the table by summing up the time spent executing eachmachine instruction that is part of the function.

In one embodiment, the performance analysis module 806 creates a tablewhich records for each task in the program the amount of time that theprocessor 110 spent executing that task during the execution of theprogram recorded in the trace data listing 213. The trace data listing213 specifies for each machine instruction execution what task wasexecuting at the time. The performance analysis module 806 fills in theentry for each task in the table by processing the trace data listing,summing up the time spent executing each machine instruction executed bythe task.

The performance analysis module 806 provides the performance statisticsfor each instruction, source line, function, and task to the debugger112. The debugger 112 displays the associated instruction, source line,function, or task on the output device 108 with an annotation indicatingthe performance statistic. The performance annotation can be a measureof time such as milliseconds or seconds or it can be a percentage orfraction of the total execution time. In one embodiment, the debugger112 receives an input for turning the display of the performance data onor off. In one embodiment, the performance data appears in a differentcolor than the source code text to indicate performance data.

FIG. 9 illustrates a performance display 900 that can be displayed onthe output device 108. The performance display lists a sequence ofinstructions with the performance data appearing in the margins asannotations to each of the associated instructions.

FIG. 10 illustrates a process 1000 for displaying a performance display.At a process block 1002, the mapping file 804 is accessed. In oneembodiment, the performance analysis module 806 translates the sourcecode instructions into machine instructions at a process block 1004. Theperformance analysis module 806 then searches the trace data listing 213until machine instructions are found at a process block 1006.Accordingly, the performance analysis module 806 determines theexecution time of the machine code instruction that was found byanalyzing the trace data listing 213 at a process block 1008. Further,the performance analysis module 806 determines the execution time of thenext machine code instruction at a process block 1010. Finally, theperformance analysis module 806 subtracts the execution time of themachine code instruction from the execution time of the next machinecode instruction to calculate performance data such as the time spent bythe processor 110 executing the instruction.

FIG. 11 illustrates a coverage analysis configuration 1100. In oneembodiment, the coverage analysis module 1102 counts how many machineinstructions are included in each source line, and how many of themachine instructions that are associated with the source line had anon-zero execution time during the time represented by the trace datalist 213, and from this it computes the percentage of machineinstructions in the source line that were executed in the timerepresented by the trace data list 213. In one embodiment, theperformance analysis module 806 counts how many machine instructions areincluded in each function, and how many of the machine instructions thatare associated with the function were executed during the timerepresented by the trace data list 213, and from this it computes thepercentage of machine instructions in the function that were executed inthe time represented by the trace data list 213. A source line orfunction could have every machine instruction executed at least once,none of its machine instructions ever executed, or some fraction of itsmachine instructions executed. In one embodiment, the performanceanalysis module 806 can perform the functions described for the coverageanalysis module 1102.

In one embodiment, a coverage display indicates whether a machineinstruction, source line, function, or task was never executed by theprocessor 110 during the period recorded by the trace data listing 213.In one embodiment, the coverage data is an annotation in a displaywindow of the source code or machine instructions, or both. Theannotation indicates for each source line or machine instruction that itwas never executed. In another embodiment, the coverage data is ahighlighting of the machine instruction or source code to indicate thatit was not executed. For example, the source code that was not executedmay have a block around it. The source code that was not executed mayalso be color highlighted to indicate that the particular machineinstruction or source code line was not executed. In one embodiment, thedebugger 112 provides the coverage data with the associated machineinstruction or source code line to the output device 108 to display thecoverage data. In one embodiment, the coverage data annotates eachsource code line or function with the percentage or fraction of themachine instructions in the source code line or function were executedor not executed. In one embodiment, the coverage data annotates eachsource code line or function for which any instruction was not executed.

FIG. 12 illustrates a coverage display 1200 that is displayed on theoutput device 108. In one embodiment, source code lines are listed inthe order they appear in the program and they are annotated withcoverage data that indicates which source code lines were not executed.In another embodiment and identification of the source code lines,machine instructions, functions, and tasks that were never executed areoutput to the display 108. In another embodiment an identification ofthe source lines for which every instruction was not executed are outputto the display 108. In another embodiment an identification of thefunctions for which every instruction was not executed are output to thedisplay 108.

The main use of the coverage display 1200 is to validate testingprocedures. If a series of tests fail to execute every machineinstruction of every source line, function, and task in a program, thenthe tests have not tested those machine instructions, source lines,functions, or tasks. This indicates that the tests do notcomprehensively test the program.

In one embodiment, an annotation is also provided to indicate theinstructions that were executed. In another embodiment, an annotation isprovided to indicate the instructions that were executed and anannotation is not provided to indicate that instructions were notexecuted.

FIG. 13 illustrates a process for displaying coverage analysis data. Ata process block 1302, the process 1300 records a sequence of machineinstructions performed by a processor as trace data. Further, at aprocess block 1304, the process 1300 accesses a mapping file. Inaddition, at a process block 1306, the process 1300 translates a sourceline into a plurality of machine instructions according to a mappingfound in the mapping file. At a process block 1308, the process 1300searches through at least a portion of the trace data to determinewhether each machine instruction in the plurality of machineinstructions has been executed. Finally, at a process block 1310, theprocess 1300 displays an indication of whether any of the machineinstructions in a plurality of machine instructions has been executed.

While the above description contains many specifics, these should not beconstrued as limitations on the scope of the invention, but rather as anexemplification of preferred embodiments thereof. The invention includesany combination or subcombination of the elements from the differentspecies and/or embodiments disclosed herein. One skilled in the art willrecognize that these features, and thus the scope of the presentinvention, should be interpreted in light of the following claims andany equivalents thereto.

We claim:
 1. A method of debugging a computer program comprising:recording trace data from an execution of the computer program for atime period, wherein said trace data comprises a record of a sequence ofinstructions performed by a processor while executing a portion of saidcomputer program, along with a read indicator and one or more affectedmemory address identifiers for one or more of said instructions that isassociated with a read operation, and a write indicator and one or moreaffected memory address identifiers for one or more of said instructionsthat is associated with a write operation; setting an access breakpointin the computer program; wherein said access breakpoint is associatedwith a memory location and is triggered upon detection of an attempt toaccess said memory location; simulating a portion of the execution ofthe computer program as recorded in the trace data in the reverse orderwith respect to the order in which the trace data was recorded untildetecting an attempt to access a memory location associated with saidaccess breakpoint; inputting a start command to continue simulating aportion of the execution of the computer program as recorded in thetrace data in the reverse order with respect to the order in which thetrace data was recorded until one of a plurality of conditions is met,wherein one of the plurality of conditions is an attempt to simulate theexecution of a machine instruction associated with one or more of theplurality of breakpoints and setting one or more additional breakpointsbefore inputting the start command.
 2. The method of claim 1, furthercomprising providing a display command before inputting the startcommand.
 3. The method of claim 2, wherein providing a display commandcomprises providing a display command to view the contents of aparticular memory location.
 4. The method of claim 2, wherein providinga display command comprises providing a display command to view avariable.
 5. The method of claim 1, further comprising inputting a startcommand to continue simulating a portion of the execution of thecomputer program as recorded in the trace data in the same order withrespect to the order in which the trace data was recorded until one of aplurality of conditions is met, wherein one of the plurality ofconditions is an attempt to simulate the execution of a machineinstruction associated with one or more of the plurality of breakpoints.6. The method of claim 5, further comprising providing a display commandbefore inputting the start command.
 7. The method of claim 6, whereinproviding a display command comprises providing a display command toview the contents of a particular memory location.
 8. The method ofclaim 6, wherein providing a display command comprises providing adisplay command to view a variable.
 9. A method of debugging a computerprogram comprising: recording trace data from an execution of thecomputer program for a time period; setting a plurality of executionbreakpoints in the computer program; simulating a portion of theexecution of the computer program as recorded in the trace data in thereverse order until one of a plurality of conditions is met, wherein oneof the plurality of conditions is an attempt to simulate the executionof a machine instruction associated with one of the plurality ofexecution breakpoints; inputting a start command to continue simulatinga portion of the execution of the computer program as recorded in thetrace data in the reverse order with respect to the order in which thetrace data was recorded until one of a plurality of conditions is met,wherein one of the plurality of conditions is an attempt to simulate theexecution of a machine instruction associated with one of the pluralityof execution breakpoints; and setting one or more additional breakpointsbefore inputting the start command.
 10. The method of claim 9, furthercomprising providing a display command before inputting the startcommand.
 11. The method of claim 10, wherein providing a display commandcomprises providing a display command to view the contents of aparticular memory location.
 12. The method of claim 10, whereinproviding a display command comprises providing a display command toview a variable.
 13. The method of claim 9, further comprising providinga display command before setting one or more additional breakpoints. 14.The method of claim 13, wherein providing a display command comprisesproviding a display command to view the contents of a particular memorylocation.
 15. The method of claim 13, wherein providing a displaycommand comprises providing a display command to view a variable.
 16. Amethod of finding an error in a computer program comprising: recordingtrace data from an execution of the computer program for a time period;setting a plurality of execution breakpoints in the computer program;simulating a portion of the execution of the computer program asrecorded in the trace data in the reverse order until one of a pluralityof conditions is met, wherein one of the plurality of conditions is anattempt to simulate the execution of a machine instruction associatedwith one of the plurality of execution breakpoints; providing a displaycommand; inputting a start command to continue simulating a portion ofthe execution of the computer program as recorded in the trace data inthe reverse order with respect to the order in which the trace data wasrecorded until one of a plurality of conditions is met, wherein one ofthe plurality of conditions is an attempt to simulate the execution of amachine instruction associated with one of the plurality of executionbreakpoints; and setting one or more additional breakpoints beforeinputting the start command.
 17. The method of claim 16, whereinproviding a display command comprises providing a display command toview the contents of a particular memory location.
 18. The method ofclaim 16, wherein providing a display command comprises providing adisplay command to view a variable.
 19. A method of finding an error ina computer program comprising: recording trace data from an execution ofthe computer program for a time period; setting a plurality of executionbreakpoints in the computer program; and simulating a portion of theexecution of the computer program as recorded in the trace data in thereverse order until one of a plurality of conditions is met, wherein oneof the plurality of conditions is an attempt to simulate the executionof a machine instruction associated with one of the plurality ofexecution breakpoints; inputting a start command to continue simulatinga portion of the execution of the computer program as recorded in thetrace data in the same order with respect to the order in which thetrace data was recorded until one of a plurality of conditions is met,wherein one of the plurality of conditions is an attempt to simulate theexecution of a machine instruction associated with one of the pluralityof execution breakpoints and setting one or more additional breakpointsbefore inputting the start command.
 20. The method of claim 19, furthercomprising providing a display command before inputting the startcommand.
 21. The method of claim 20, wherein providing a display commandcomprises providing a display command to view the contents of aparticular memory location.
 22. The method of claim 20, whereinproviding a display command comprises providing a display command toview a variable.
 23. The method of claim 19, further comprisingproviding a display command before setting one or more additionalbreakpoints.
 24. The method of claim 23, wherein providing a displaycommand comprises providing a display command to view the contents of aparticular memory location.
 25. The method of claim 23, whereinproviding a display command comprises providing a display command toview a variable.
 26. A method of debugging a computer programcomprising: recording trace data from an execution of the computerprogram for a time period, wherein said trace data comprises a record ofa sequence of instructions performed by a processor while executing aportion of said computer program, along with a read indicator and one ormore affected memory address identifiers for one or more of saidinstructions that is associated with a read operation; setting an accessbreakpoint in the computer program; wherein said access breakpoint isassociated with a memory location and is triggered upon detection of anattempt to access said memory location; simulating a portion of theexecution of the computer program as recorded in the trace data in thereverse order with respect to the order in which the trace data wasrecorded until detecting an attempt to access a memory locationassociated with said access breakpoint; inputting a start command tocontinue simulating a portion of the execution of the computer programas recorded in the trace data in the reverse order with respect to theorder in which the trace data was recorded until one of a plurality ofconditions is met, wherein one of the plurality of conditions is anattempt to simulate the execution of a machine instruction associatedwith one or more of the plurality of breakpoints and setting one or moreadditional breakpoints before inputting the start command.
 27. Themethod of claim 26, further comprising providing a display commandbefore inputting the start command.
 28. The method of claim 27, whereinproviding a display command comprises providing a display command toview the contents of a particular memory location.
 29. The method ofclaim 27, wherein providing a display command comprises providing adisplay command to view a variable.
 30. The method of claim 26, furthercomprising inputting a start command to continue simulating a portion ofthe execution of the computer program as recorded in the trace data inthe same order with respect to the order in which the trace data wasrecorded until one of a plurality of conditions is met, wherein one ofthe plurality of conditions is an attempt to simulate the execution of amachine instruction associated with one or more of the plurality ofbreakpoints.
 31. The method of claim 30, further comprising providing adisplay command before inputting the start command.
 32. The method ofclaim 31, wherein providing a display command comprises providing adisplay command to view the contents of a particular memory location.33. The method of claim 31, wherein providing a display commandcomprises providing a display command to view a variable.
 34. A methodof debugging a computer program comprising: recording trace data from anexecution of the computer program for a time period, wherein said tracedata comprises a record of a sequence of instructions performed by aprocessor while executing a portion of said computer program, along witha write indicator and one or more affected memory address identifiersfor one or more of said instructions that is associated with a writeoperation; setting an access breakpoint in the computer program; whereinsaid access breakpoint is associated with a memory location and istriggered upon detection of an attempt to access said memory location;simulating a portion of the execution of the computer program asrecorded in the trace data in the reverse order with respect to theorder in which the trace data was recorded until detecting an attempt toaccess a memory location associated with said access breakpoint;inputting a start command to continue simulating a portion of theexecution of the computer program as recorded in the trace data in thereverse order with respect to the order in which the trace data wasrecorded until one of a plurality of conditions is met, wherein one ofthe plurality of conditions is an attempt to simulate the execution of amachine instruction associated with one or more of the plurality ofbreakpoints and setting one or more additional breakpoints beforeinputting the start command.
 35. The method of claim 34, furthercomprising providing a display command before inputting the startcommand.
 36. The method of claim 35, wherein providing a display commandcomprises providing a display command to view the contents of aparticular memory location.
 37. The method of claim 35, whereinproviding a display command comprises providing a display command toview a variable.
 38. The method of claim 34, further comprisinginputting a start command to continue simulating a portion of theexecution of the computer program as recorded in the trace data in thesame order with respect to the order in which the trace data wasrecorded until one of a plurality of conditions is met, wherein one ofthe plurality of conditions is an attempt to simulate the execution of amachine instruction associated with one or more of the plurality ofbreakpoints.
 39. The method of claim 38, further comprising providing adisplay command before inputting the start command.
 40. The method ofclaim 39, wherein providing a display command comprises providing adisplay command to view the contents of a particular memory location.41. The method of claim 39, wherein providing a display commandcomprises providing a display command to view a variable.
 42. A methodof debugging a computer program comprising: recording trace data from anexecution of the computer program for a time period, wherein said tracedata comprises a record of a sequence of instructions performed by aprocessor while executing a portion of said computer program; setting anaccess breakpoint in the computer program; wherein said accessbreakpoint is associated with a memory location and is triggered upondetection of an attempt to access said memory location; simulating aportion of the execution of the computer program as recorded in thetrace data in the reverse order with respect to the order in which thetrace data was recorded until detecting an attempt to access a memorylocation associated with said access breakpoint; inputting a startcommand to continue simulating a portion of the execution of thecomputer program as recorded in the trace data in the reverse order withrespect to the order in which the trace data was recorded until one of aplurality of conditions is met, wherein one of the plurality ofconditions is an attempt to simulate the execution of a machineinstruction associated with one or more of the plurality of breakpointsand setting one or more additional breakpoints before inputting thestart command.
 43. The method of claim 42, further comprising providinga display command before inputting the start command.
 44. The method ofclaim 43, wherein providing a display command comprises providing adisplay command to view the contents of a particular memory location.45. The method of claim 43, wherein providing a display commandcomprises providing a display command to view a variable.
 46. The methodof claim 42, further comprising inputting a start command to continuesimulating a portion of the execution of the computer program asrecorded in the trace data in the same order with respect to the orderin which the trace data was recorded until one of a plurality ofconditions is met, wherein one of the plurality of conditions is anattempt to simulate the execution of a machine instruction associatedwith one or more of the plurality of breakpoints.
 47. The method ofclaim 46, further comprising providing a display command beforeinputting the start command.
 48. The method of claim 47, whereinproviding a display command comprises providing a display command toview the contents of a particular memory location.
 49. The method ofclaim 47, wherein providing a display command comprises providing adisplay command to view a variable.